Which particle system is best for trees in a forest?
I've always used Emitter to populate a large number of static objects. I don't think hair is needed here.

In the particles panel:


Set the start and end value to -1, and the lifetime to some high value that's longer than your total animation. This avoids any popping in and out.
Turn off particles newtonian physics completely.
Use object instancing or group instancing to replace each particle with a tree
Play with other settings (variable size, initial rotation, etc)


Cycles is surprisingly good at rendering large number of instanced geometry. So unless performance is really important, rending actual geometry shouldn't be too bad. And lastly, this tutorial might help.